(1) Field of the invention
The present invention relates to a composite photosensitive element for use in electrophotography and a process of forming images, in particular a composite photosensitive element for use in electrophotography capable of achieving a dichromatic reproduction even from a multi-color original by a single exposure step not to mention a monochromatic (black and white) reproduction and a dichromatic reproduction process using same.
(2) Description of the prior art
As the prior art electrophotography there is well known one designed so as to form two electrostatic latent images having a polarity opposite to each other which comprises sensitizing a photosensitive element consisting of an electrically conductive substrate and two photoconductive layers, laminated thereon, whose photosensitive wavelength range is different from each other, by subjecting said element to a primary charge and a secondary charge of a polarity opposite to the former and exposing a different colored original image simultaneously or successively under this state.
As the original used herein there is normally enumerated one having a black and chromatic (A) image on a white ground (background area), wherein said chromatic (A) is mostly red. Accordingly, in the case of the photosensitive element of this sort, the one photosensitive layer is made of a material having a sensitivity to the chromatic light (A) and the other photosensitive layer is made of a material which has no sensitivity to the chromatic light (A) or may be made of a raw material having a sensitivity to the chromatic light (A) on condition that a filter layer is provided.
As the photosensitive element (composite element for use in electrophotography) as mentioned above, there has been proposed a photosensitive element wherein a photosensitive layer (second photosensitive layer) more remote from an electrically conductive substrate is used as a layer having a sensitivity to said chromatic light (A), said second photosensitive layer comprising the lamination of a relatively thin charge carrier generating layer and a relatively thick charge transport layer.
In other words, as one of the typical photosensitive elements of this type there is well known a photosensitive element wherein a first photoconductive layer (contacting directly on an electrically conductive substrate) is formed as an amorphous vapordeposited layer of Se or Se-Te alloy or a laminated layer (Se/SeTe) comprising a Se vapordeposited layer and a Se-Te vapor-deposited layer laminated thereon as a sensitizing layer, while a second photoconductive layer consists essentially of azo pigment. The composite photosensitive element of this sort is characterized in that the less the electric potential of the first photoconductive layer decays at the time of exposure to a red color in the reproduction process, the higher the density of a reproduced copy is.
However, this conventional composite photosensitive element is defective in that the Se layer or the Se/SeTe laminated layer has a sensitivity even to a long wavelength more than about 700 nm due to the presence of crystalline Se, and therefore only the second photoconductive layer or the filter effect of copper phthalocyanine added to an intermediate layer provided in case of need can not prevent the electric potential drop at the time of exposure to a red color completely or almost completely. The red color in "the time of exposure to a red color" means a light having a spectral distribution only in the range of 560-600 nm or more.
Accordingly, when it is absolutely required that the first photoconductive layer should scarcely have a sensitivity to radiation of the above mentioned red light, said requirement may be satisfied by the ways of (a) selectively finding the materials for use in the first photoconductive layer which have a sensitivity to only the wavelength range less than the above defined one and (b) increasing the red color cutting faculty of the intermediate layer by adding much more of blue pigment such as copper phthalocyanine or the like or blue dye to the intermediate layer. In this connection, it is to be noted that FIG. 5 illustrates the spectral permeability of the intermediate layer added with copper phthalocyanine. It may be understood from this graph that in the above mentioned (b) the first photoconductive layer may have a sensitivity to the range of 600-700 nm. However, this composite photosensitive element also involves undesirable aspects that the selection of raw materials as mentioned in the preceding (a) is not necessarily easy, and in the preceding (b) the quantity of light reaching the first photoconductive layer is reduced largely and consequently the residual potential is increased.
The fact is that Se or SeTe alloy has been considered to be adaptable for the first photoconductive layer of the photosensitive element for use in dichromatic reproduction as described above, since the durability of the Se photosensitive element is superior more than any other inorganic or organic photosensitive elements at the present stage, the sensitivity of simple substance Se (non-crystalline Se) is mainly in the range of less than 560 nm, and further the total sensitivity or spectral permeability of the element may be changed relatively easily by laminating the SeTe sensitizing layer on the Se layer as mentioned above and regulating the content of Te. In spite of this, the use of the Se layer or Se/SeTe laminated layer as the first photoconductive layer was found undesirable in the following points that it sometimes produced good results and sometimes produced bad results and consequently the produced composite photosensitive elements per se were extremely unbalanced in the quality and unreliable. This unbalanced quality is considered to result from that the degree of crystallization of selenium or selenium-tellurium alloy changes depending on the conditions under which said metal or alloy is vapordeposited.